An important challenge in biomedical research is to non-invasively characterize tissue conditions, for the purposes of diagnosis of disease, such as (pre)cancerous conditions, or for monitoring response to treatment. Non-invasive optical reflectance measurements have been used to extract diagnostic information from a variety of tissue types, in vivo, such as colon49, skin50, brain51, cervix52, esophagus53, prostate54 and bronchial mucosa55. The experimental setup of these systems often consists of a fiber probe with a diameter of a few millimeters or less, with one or more fibers that transmit light to and from the tissue. These in situ measurements of the optical properties of a tissue can reveal information concerning the morphological and biochemical composition of the tissue. Optical techniques are also used in vivo to identify changes that occur in biological tissues1-3 associated with disease progression.
Tissues can be characterized by their optical properties, which are defined by the absorption coefficient (μa), the scattering coefficient (μs), the phase function (p(θ)), the anisotropy value (g=<cos θ>) and the reduced scattering coefficient (μs′=μs(1−g)). The diffusion approximation to the Boltzman transport equation is a method that has been used successfully to determine the absorption coefficient and the reduced scattering coefficient in turbid media4-6. The validity of the diffusion approximation is limited to media with higher scattering than absorption (μs′>>μa), which is satisfied in biological tissues for the wavelength region between 600-900 nm, and to large separations between the source and the detector (ρ>>1/μs′). As a consequence, the collected photons travel through a large volume of tissue, and the extracted optical properties represent average values for the tissue volume probed. However, many clinical settings require small fiber probes (e.g. endoscope working channels typically have a diameter of <3 mm). Different methods have been used to determine the optical properties in turbid media at small source-detector separations7-19, including some based on the diffusion approximation20,21. Various fiber optic probe designs have been developed for reflectance and fluorescence measurements22-25. In many applications the tissues of interest are thin. Most cancers arise in the epithelium, which is a superficial tissue layer with a thickness, typically, of 100-500 micrometers. Hence, sensitivity to the optical properties of the epithelial layer requires superficial measurement techniques. The penetration depth of the collected photons depends on the fiber probe geometry and the optical properties of the sample16,24,26,27, which may allow the interrogation depth to extend beyond the epithelial layer.